1. Field of the Invention
This invention relates to a control system for a direct injection spark ignition internal combustion engine, more particularly to a system for controlling a direct injection spark ignition engine connected to an automatic transmission that modifies the engine power in accordance with a selected one of shift programs.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 2000-161098 teaches a system for controlling a lean-burn internal combustion engine, not a direct injection spark ignition engine, in which the lean-burn region wherein a desired air/fuel is set to be lean, is varied when the shift programs of a normal mode for improved fuel economy and a power mode for improved performance are switched therebetween, so as to suppress a shock due to torque difference caused by the switching. To be more specific, in the power mode, the lean-burn region is made narrower than that in the normal mode, or the desired air/fuel itself is made richer than that of the normal mode partially at a high load and a high engine speed or throughout the engine loads and speeds.
In this prior art, since, however, the desired air/fuel ratio in the power mode is made relatively richer than that in the normal mode, the fuel economy is disadvantageously degraded when the mode is switched to the power mode. Moreover, since the desired air/fuel ratio is immediately changed in response to the switching to the power mode, in other words, since the torque required by the engine is not determined in response to this switching, in addition to the degradation of fuel economy, acceleration is not so satisfactory, depending on the operating conditions, as expected by the vehicle operator.
An object of the present invention is therefore to provide a control system for a direct injection spark ignition internal combustion engine connected to an automatic transmission that modifies the engine torque in accordance with a selected one of shift programs, which determines the torque required by the engine in response to the switching of the shift programs and based thereon, determines an operation mode including a desired air/fuel ratio, thereby enabling to produce the acceleration as expected by the vehicle operator throughout the operating conditions with less degradation of fuel economy, when the shift program is switched to one for improved performance.
In order to achieve the object, there is provided a system for controlling an internal combustion engine in which gasoline fuel is directly injected into cylinder combustion chambers and is ignited by a spark plug, and an automatic transmission connected to the engine to modify engine power in accordance with a selected one of shift programs, comprising: engine operating condition detecting means for detecting operating conditions of the engine including at least an engine speed and an engine load; required torque determining means for determining a torque required by the engine based on the detected engine speed and the engine load in accordance with ones, of characteristics predetermined separately for the shift programs, that are corresponding to the selected shift program; operation mode determining means for determining an operation mode of the engine to one of at least three operation modes comprising a stoichiometric air/fuel ratio operation mode in which a desired air/fuel ratio to be supplied to the engine is set to a stoichiometric air/fuel ratio and lean-burn operation modes including at least a pre-mixture combustion operation mode in which the desired air/fuel ratio is set to be leaner than the stoichiometric air/fuel ratio and a stratified combustion operation mode in which the desired air/fuel ratio is set to be leaner than that of the pre-mixture combustion operation mode, based on at least the determined required torque; output fuel injection amount determining means for determining an output fuel injection amount to be supplied to the engine by correcting a basic fuel injection amount, determined based on the detected operating conditions, by the set desired air/fuel ratio of the determined operation mode; and fuel injector which directly injects fuel into the combustion chamber of the engine determined based on the output fuel injection amount.